The reproductive epidemiology program emphasizes the development and application of new methods for measuring and analyzing human reproductive outcomes. Such outcomes include fertility, sub-clinical early fetal loss, spontaneous abortion, fetal growth, and birthweight. Each of these outcomes can be affected by environmental factors, and represents a possible endpoint for studying the effects of toxins on human reproduction. One major component of this project is a prospective study of early fetal loss among 300 women. A recently developed urine assay for human chorionic gonadotropin is being used to estimate the risk of early loss among these women. Daily urine specimens are being collected from women who have discontinued their use of birth control in order to become pregnant. This year the pilot study of 30 women has been completed and enrollment of the full sample is in progress. Risk of early loss will be studied in relation to common exposures in this population, such as use of alcohol, tobacco, caffeine beverages and medications. Another area of interest is the possible usefulness of measuring fertility through retrospective estimates of time to pregnancy. A feasibility study of this approach is under way. Work also continues on more theoretical problems in the analysis of spontaneous abortion risk and the analysis of birth weight.